Implement the hybrid clipboard with deferred cut

⌘X/⌘C/⌘V for cards and lanes per 04-interactions.md § Clipboard:

- ClipboardStore stages full folder snapshots eagerly at the gesture into
  Application Support (at most the current copy; sweep at launch and on
  each copy purges what the pasteboard no longer references; a copy made
  before quitting pastes whole after restart) and writes the pasteboard a
  JSON manifest — every entry embedding its index.md, lane entries their
  cards' too — plus plain-text titles.
- Cut is Finder-style deferred: items dim in place off pendingCut, void on
  pasteboard takeover (changeCount, no timers), source-board close, or
  per-item external tombstoning; the first armed paste moves the surviving
  originals whole (tombstoned interior cards land in the destination's
  trash), a second paste materializes copies from staging.
- Paste anchors by the shared flatten-order rule (NewCardTarget's anchor,
  extracted); a tombstoned selection never anchors; lane paste reaches the
  right end and stays enabled on a zero-lane board; paste into the source
  board is the within-board lane duplicate; copies keep created, take
  fresh GUIDs, and strip tombstoned cards; trash-sourced copies strip
  deleted: at materialization; ⌘X is disabled on the trash side.
- A degraded paste is loud, never silent: staging gone → the embedded
  index.md fallback lands content-intact, attachments absent, and a
  BannerCenter-phrased row names what was lost.
- The standard Edit items validate through conditionally-attached
  onCommand handlers, so AppKit's enablement mirrors the availability
  predicates; text fields keep their own clipboard while focused.

879 unit tests (68 new).

Claude-Session: https://claude.ai/code/session_01SR4XGjmBE16ZUYWpfFHXwY

Kanban/UI/Board/ClipboardCommands.swift

@@ -0,0 +1,78 @@
+import AppKit
+import SwiftUI
+
+// MARK: - The Edit menu's clipboard row
+
+/// Edit ▸ Cut / Copy / Paste (⌘X / ⌘C / ⌘V) on the board — 11-command-nexus.md's Edit row, whose
+/// scope is "Board window: cards and lanes … in the trash, ⌘C copy-out only (card and lane entries),
+/// ⌘X disabled … text editors: standard text clipboard".
+///
+/// ### Why this is a responder answer and not three menu items
+///
+/// **Select All's precedent, exactly** (`BoardView`): the standard Edit menu already carries these
+/// three, and AppKit dispatches `cut:`/`copy:`/`paste:` down the responder chain, so the board
+/// answers them as a responder. Adding items of our own would put a second "Copy"-titled row in the
+/// menus, which titles-are-API forbids outright (04-interactions.md ▸ Configurable bindings) — a
+/// custom binding is stored against a title, and two rows sharing one would be ambiguous.
+///
+/// ### Availability is the handler's presence
+///
+/// `onCommand(_:perform:)` takes an **optional** action, and a `nil` action means the view does not
+/// respond to that selector at all — which is precisely what AppKit's automatic menu validation
+/// reads. So attaching the handler conditionally *is* the validation: there is one condition per
+/// command, it decides both whether the item is enabled and whether the gesture does anything, and
+/// the two can never disagree because they are the same expression.
+///
+/// The conditions themselves live on `ClipboardStore` (`canCopy`/`canCut`/`canPaste`), beside the
+/// gestures they gate, for the reason every rule in this codebase that can be a named predicate is
+/// one: an item that is going to no-op should not look available.
+///
+/// ### The focused-editor rule, twice over
+///
+/// A focused text field consumes these selectors natively, so ⌘X/⌘C/⌘V inside an inline title editor
+/// stay text operations without anything here doing the arithmetic. The predicates still refuse while
+/// an editor is open (04 ▸ Grammar: "board-scoped menu commands … disable via menu validation"),
+/// which is belt over braces — but a board command that stayed armed under an editor is exactly the
+/// fall-through 04's fixed grammar is careful to rule out.
+extension View {
+
+    /// Attaches the board's clipboard responders, each only while its command applies.
+    func boardClipboardCommands(store: BoardStore, clipboard: ClipboardStore) -> some View {
+        self
+            .onCommand(#selector(NSText.cut(_:)), perform: clipboard.canCut(from: store) ? {
+                clipboard.cut(from: store)
+            } : nil)
+            .onCommand(#selector(NSText.copy(_:)), perform: clipboard.canCopy(from: store) ? {
+                clipboard.copy(from: store)
+            } : nil)
+            .onCommand(#selector(NSText.paste(_:)), perform: clipboard.canPaste(into: store) ? {
+                clipboard.paste(into: store)
+            } : nil)
+    }
+}
+
+// MARK: - The deferred cut's treatment
+
+extension View {
+
+    /// **Cut items dim in place until paste moves them** (04-interactions.md ▸ Clipboard).
+    ///
+    /// The same reduced opacity a trash row wears while it is being dragged, and for the same reason:
+    /// the item is still there, still selectable, still the user's — it is simply spoken for. A cut
+    /// item is deliberately *not* lifted out of the layout the way a dragged one is; a Finder-style
+    /// deferred cut promises the board looks unchanged until the paste lands.
+    ///
+    /// Membership is read straight off `TransientBoardState.pendingCut`, which is where the rules
+    /// already live: a reload ejects a tombstoned or vanished member (so a deleted cut card undims by
+    /// itself), and `ClipboardStore` clears the set outright when the cut is consumed or voided.
+    func cutTreatment(of id: ItemID, in store: BoardStore) -> some View {
+        opacity(store.transient.pendingCut.ids.contains(id) ? ClipboardTreatment.dimmedOpacity : 1)
+    }
+}
+
+/// The one number the cut's treatment is (03-board-ui.md § Motion keeps every duration and curve in
+/// `Motion`; this is neither, but it is the same "no literal at a call site" rule applied to the one
+/// value three views share).
+enum ClipboardTreatment {
+    static let dimmedOpacity: Double = 0.45
+}